Angle of Sun and Solar Radiation
What is the relation between the angle of the noon sun and
the quality of solar radiation received per square centimeter at the
outer edge of the atmosphere?
What causes the intensity and duration of solar radiation received
at any place to vary throughout the year?
The angle of the noon sun will not affect the average intensity so much
as where the intensity is greatest. When the angle of the noon sun is
more northward, the northern hemisphere receives most of the radiation.
This is why we experience summer at this time. When the sun is very
much toward the south, the radiation intensity is greater in the
Although tilt is not a direct affect to intensity, distance from the sun
and sun activity are very important. The Earth's orbit is not a circle.
The Earth is closest to the Sun in January. It is furthest six months
later. The entire Earth receives a greater radiation during January.
It receives less radiation during July. In addition, the Sun itself is
not constant: sometimes the sun emits more radiation, sometimes less.
This can also affects the amount of radiation that reaches our planet.
I suspect the only measurable link between tilt and radiation intensity
is the fact that Sun appears furthest South when the Earth closest to
the Sun. Thus, more radiation strikes the Earth when the Sun is
furthest South. A lower radiation intensity strikes the atmosphere
above Illinois, but more strikes the atmosphere of entire planet.
Dr. Ken Mellendorf
Illinois Central College
these are actually quite distinct questions, and there are many
"What is the relation between the angle of the noon sun and the
quality of solar radiation received per square centimeter at the outer edge
of the atmosphere?"
I am not sure what you mean by 'quality'. If you mean _quantity_, the
simplest relationship is that the radiation is lowered, approximately
proportionally to the sine of the angle of the light. The more extreme
the angle, the less the apparent intensity.
If terms of _quality_ of radiation (e.g. relative intensities of
different wavelengths), I am not sure angle would play a role (other
than in some negligible quantum effects). The earth's magnetic field
also plays a role in deflecting some radiation (which would affect
radiation quality) and the magnetic field does have an orientation,
but that orientation is not independent of the earth's orientation
(and therefore not strictly consistent with angle of the surface)
You specifically say the outer edge of the atmosphere, but I also want
to discuss the role the atmosphere plays, since it affects _quality_
as well. In terms of _quality_, the atmosphere filters out / reduces
some wavelengths of light (UV, for example), and it also redirects
different wavelengths of visible light (read about Rayleigh
scattering). Especially for Rayleigh scattering, the scattering is
very much dependent on the angle of the light.
"What causes the intensity and duration of solar radiation received at any
place to vary throughout the year?"
The biggest impact on intensity and duration of solar radiation is
cloud cover -- cloudy areas get less sun. The intensity of the sun
changes as well, but in general those changes are not as rapid as
cloud cover changes.
There are also differences as the times of sunrise and sunset change. (nau)
Hope this helps,
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Update: June 2012